refactor(time): one timestamp formatter honouring the clock/date settings (#139)

Audit of every rendered time found five families of ad-hoc formatting:
the shared Time component + copies of its today/yesterday branch
(forwarded header, thread summary, read receipts, device tile, moderation
alerts, edit history), locale-default toLocale*String calls that ignored
the user's 12/24 h and date-format settings (scheduled tray, reminders,
schedule preview, notification snooze, bookmarks, threads list, search
cache line, room insights, media gallery), a hard-coded en-US date in the
activity log, and three relative-age variants.

utils/formatTimestamp.ts now holds the rules — today → time; yesterday /
tomorrow → day word + time; last 6 days → weekday + time; older → date +
time in dateFormatString — plus autoDate / time / date / dateTime styles,
formatDayDivider (full weekday), formatShortAge (room list) and
formatRelativeAge (list rows). useTimestampFormatter binds them to the
settings. 11 unit tests with an injected 'now'.

Visible changes are limited to consistency: 12 h times keep the existing
zero-padded hh:mm A; the a11y label and Created-by line use the user's
date format instead of a fixed long month.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/features/settings/devices/DeviceTile.tsx

@@ -20,15 +20,13 @@ import FocusTrap from 'focus-trap-react';
 import { IMyDevice, MatrixError } from 'matrix-js-sdk';
 import { SettingTile } from '../../../components/setting-tile';
 import { useMatrixClient } from '../../../hooks/useMatrixClient';
-import { timeDayMonYear, timeHourMinute, today, yesterday } from '../../../utils/time';
+import { useTimestampFormatter } from '../../../hooks/useTimestampFormatter';
 import { BreakWord } from '../../../styles/Text.css';
 import { AsyncStatus, useAsyncCallback } from '../../../hooks/useAsyncCallback';
 import { SequenceCard } from '../../../components/sequence-card';
 import { SequenceCardStyle } from '../styles.css';
 import { LogoutDialog } from '../../../components/LogoutDialog';
 import { stopPropagation } from '../../../utils/keyboard';
-import { useSetting } from '../../../state/hooks/settings';
-import { settingsAtom } from '../../../state/settings';
 
 export function DeviceTilePlaceholder() {
   return (
@@ -43,20 +41,14 @@ export function DeviceTilePlaceholder() {
 }
 
 function DeviceActiveTime({ ts }: { ts: number }) {
-  const [hour24Clock] = useSetting(settingsAtom, 'hour24Clock');
-  const [dateFormatString] = useSetting(settingsAtom, 'dateFormatString');
+  const { format } = useTimestampFormatter();
 
   return (
     <Text className={BreakWord} size="T200">
       <Text size="Inherit" as="span" priority="300">
         {'Last activity: '}
       </Text>
-      <>
-        {today(ts) && 'Today'}
-        {yesterday(ts) && 'Yesterday'}
-        {!today(ts) && !yesterday(ts) && timeDayMonYear(ts, dateFormatString)}{' '}
-        {timeHourMinute(ts, hour24Clock)}
-      </>
+      {format(ts)}
     </Text>
   );
 }

src/app/features/settings/notifications/SystemNotification.tsx

@@ -2,6 +2,7 @@ import React, { useCallback, useEffect, useState } from 'react';
 import { Box, Text, Switch, Button, Chip, Icon, Icons, color, config, Spinner } from 'folds';
 import { IPusherRequest } from 'matrix-js-sdk';
 import { useAtomValue, useSetAtom } from 'jotai';
+import { useTimestampFormatter } from '../../../hooks/useTimestampFormatter';
 import { NOTIFICATION_SOUND_MAP } from '../../../utils/notificationSounds';
 import { SequenceCard } from '../../../components/sequence-card';
 import { SequenceCardStyle } from '../styles.css';
@@ -139,6 +140,7 @@ const SNOOZE_PRESETS: Array<{ label: string; resolve: (now: number) => number }>
 // Cross-platform "pause notifications" — sets a snooze instant that the
 // notification gate (ClientNonUIFeatures) reads to suppress alerts + sounds.
 function PauseNotifications() {
+  const { prefs } = useTimestampFormatter();
   const snoozeUntil = useAtomValue(notificationSnoozeUntilAtom);
   const setSnoozeUntil = useSetAtom(notificationSnoozeUntilAtom);
   // While paused, tick so the status flips to "on" the moment the snooze lapses.
@@ -155,7 +157,7 @@ function PauseNotifications() {
     ? 'Notifications are on.'
     : snoozeUntil >= SNOOZE_INDEFINITE
       ? 'Paused until you resume.'
-      : `Paused until ${formatFriendlyDateTime(snoozeUntil)}.`;
+      : `Paused until ${formatFriendlyDateTime(snoozeUntil, prefs)}.`;
 
   return (
     <SettingTile